The present invention relates to instrument gyros of the high precision type such as, for example, those used in relation to a reference or origin point. The present invention relates particularly to a gyroscope motor control system, which minimizes motor on-off drift errors by an amplitude modulation of the motor excitation voltage.
A long-standing problem in the operation of instrument gyros has been drift error. In addition to the usual calculated gyro precession or normal drift, so-called "random drift" errors are also present. Some of these errors are unavoidable such as those due to basic bearing resistance; other random type drift errors appear to be characteristic of the type of gyro in question.
Such a drift error appears to be characteristic of most solid rotor synchronous motor gyroscopes. Here, it was found that when the gyro was initially turned on and the rotor had become electrically locked in synchronism with the field, a small drift error that remained uniform or constant during rotor-field lock, was produced in addition to the basic random drift; however, this error although constant during a given synchronous operation of the gyro, tended to be different in an apparently random manner for each subsequent on-off operation of the gyro.
A prior art motor control system for minimizing such drift error is shown in U.S. Pat. No. 3,702,569, which is assigned to the same assignee as this application. In such a prior art motor control system, a phase shift in the motor excitation voltage is produced which results in a relative slip of the field relative to the rotor.
One problem with such prior art motor control system is that it requires a relatively high power input. Another problem with such prior art system is that it develops a relatively high temperature around the motor causing a relatively high temperature gradient inside the gyro. Another problem with such prior art system is that if the phase advance is set at a relatively small angle, the rotor does not fall out of synchronism with the field but tends to hunt about the new position; and if the phase angle is set at a relatively large angle, the rotor does not have a smooth and well defined retardation relative to the field. Another problem with such prior art system is that it causes current spikes when its abrupt frequency change occurs, which interfere with the connecting circuits.